How we calculate your rainwater harvesting estimate
Our rainwater harvesting (RWH) estimate follows the BIS IS 15797:2008 standard. Below is every formula, constant, and source. It is a planning model, not a quote — your vendor's site survey confirms the real numbers.
How much rain your roof catches
The core harvest formula multiplies your roof area by how much rain falls, the share of that rain a roof of your material actually sheds, and two practical loss factors.
annual harvest (L) = roof area (m²) × runoff coefficient × annual rainfall (mm) × first-flush factor × filter efficiency
One millimetre of rain on one square metre of roof is exactly one litre — that is why the units work out directly to litres.
Runoff coefficient — your roof material
Not all rain that lands on a roof reaches the tank. Rougher and more absorbent surfaces lose more. We use BIS-aligned coefficients:
- Metal sheet — 0.92
- Sloped tile — 0.75
- Flat RCC (the typical Indian home) — 0.70
- Other / mixed — 0.65
Loss factors
Two adjustments keep the estimate realistic:
- First-flush loss — 10%. The first runoff of each storm is diverted because it carries roof dust and debris, so we keep 90%.
- Filter efficiency — 95%. A small share is lost across the filter media.
Rainfall data
We use city-level annual rainfall for Karnataka and Tamil Nadu. If we do not have your exact city, we fall back to the nearest covered city in your state. Rainfall varies year to year — the figure is a long-run average, not a forecast.
Sizing the system
We recommend a storage tank and, for hybrid systems, recharge pits, based on your harvest volume and roof area. Tank sizing respects sensible bounds — 2,000 L minimum, up to 10,000 L for a home and up to 200,000 L for a society. Each recharge pit is modelled at 5,000 L capacity.
system cost ≈ ₹25,000 base + ₹200 per m² of roof (societies ×1.2) + ₹10,000–15,000 per recharge pit
What you save
Savings come from two sources: water you no longer buy from tankers, and the BWSSB/CMWSSB non-compliance penalty you avoid. Because roughly three-quarters of the harvest arrives in a few monsoon months, a storage tank can only be usefully refilled so many times a year — we cap savings at a realistic turnover rather than assuming the tank fills endlessly.
Tanker rates are modelled at about ₹0.105/L (Karnataka) and ₹0.108/L (Tamil Nadu), based on a ₹630 median for a 6,000 L tanker. The 10-year projection uses this as the baseline.
Mandate status
Both Karnataka and Tamil Nadu have residential RWH mandates. We surface your mandate status and link to the official regulation with its last-verified date — so you can confirm the rule rather than take our word for it.
What this estimate is not
It is a planning model. Catchment routing, plumbing runs, soil percolation rate, and your plot layout all move the real number. Use it to scope the project and budget — then get a vendor site survey.